As is known in the art, micropumps have rapidly expanded micro-hydraulic systems into a wider range of applications, such as drug delivery, chemical analysis and biological sensing. Empirical research has shown that micropumps suffer most from their extremely low efficiency.
In terms of actuation principles, the mechanical methods include piezoelectric, bimetallic, thermo-pneumatic, electrostatic, electromagnetic actuation and shape memory alloy (SMA). The non-mechanical methods include magneto-hydrodynamic (MHD), electro-hydrodynamic (EHD), and electro-osmotic actuation.
Piezoelectric actuation has been commonly used in reciprocating micropumps. This actuation concept is based upon the piezoelectric effect which correlates mechanical deformation and electrical polarization. Due to the fast response and precise dosage, piezoelectric micropumps are often used to maintain therapeutic efficacy, such as drug delivery. However, the drawbacks for the piezoelectric micropumps are considered to be the high actuation voltage and the mounting procedure.
Thermo-pneumatic micropumps are designed by a periodic change in the volume of the chamber expanded and compressed by a pair of heater and cooler. Micromachining, either for the heater and cooler or the diaphragm; contributes to the realization of this principle. The crucial disadvantages for thermo-pneumatic micropumps is the long thermal relaxation time constant of the cooling process which will limit the bandwidth of the actuation, and the driving power which is required to be maintained at a specified-constant level.
Shape memory alloy (SMA) micropumps generally refer to those applying the shape memory effect (SME) of an SMA (e.g., Titanium/Nickel (TiNi)), resulting in large pumping rates and high operating pressures. The main disadvantages of this approach are the relatively high power consumption indicating a low efficiency and the uncontrollable deformation of SMA due to its temperature sensitivity.
In an embodiment, considering the efficiency of micro-hydraulic systems, all types of pumps described above suffer from a low efficiency. Typically, the overall efficiency of a micro-pump is determined by the product of four components: volumetric efficiency, hydraulic efficiency, mechanical efficiency and electrical efficiency. Volumetric losses and hydraulic losses dominate at small scales, although an acceptable efficiency for macro-pumps has already been achieved. As the size of the system decreases, the volumetric efficiency is dramatically affected since the same dimensional and geometric tolerance result in a larger dimension fraction. In terms of hydraulic efficiency, a Reynolds number also decreases as the characteristic length scales decreases, leading to larger viscous losses. Especially at low pressure, the efficiency of all types of micro-pumps is quite low.